Phototrophy & Phototrophic Microorganisms

 Microorganisms derive energy not only from the oxidation of inorganic and organic compounds, but also from light energy, which they capture and use to synthesize ATP and reduce power (e.g., NADPH).

The process by which light energy is trapped and converted to chemical energy is called photosynthesis. Photosynthesis is one of the most significant metabolic processes on Earth because almost all our energy is ultimately derived from solar energy. It provides photosynthetic organisms with the ATP and reducing power necessary to synthesize the organic material required for growth. In turn these organisms serve as the base of most food chains in the biosphere.  

Photosynthesis carried out by a variety of organisms, both eucaryotic and bacterial, is also responsible for replenishing our supply of O₂.

 We mostly associate photosynthesis with the larger, more obvious plants, but over half the photosynthesis on Earth is carried out by microorganisms.

 Photosynthesis as a whole is divided into two parts. In the light reactions light energy is trapped and converted to chemical energy. This energy is then used to reduce or fix CO₂ and synthesize cell constituents in the dark reactions.

 Phototrophic Microorganisms 

Phototrophic organisms are a diverse group of organisms which  carry out photosynthesis. They synthesize their own food and provide energy and nutrients to other organisms. They are important in a variety of ecological and biogeochemical processes and play a vital role in our ecosystem.

 Phototrophic microorganisms are broadly classified as either oxygenic or anoxygenic, based on their ability to produce oxygen during photosynthesis. Major groups include Cyanobacteria (oxygenic) and various anoxygenic bacteria, such as purple bacteria (Rhodospirillineae), green bacteria (Chlorobiineae), and aerobic anoxygenic phototrophs (AAPs). 

 Beyond their photosynthetic pathway, microorganisms are grouped by how they obtain carbon: 

·        Photoautotrophs: 

Use sunlight to convert carbon dioxide into organic compounds, making their own food. This includes most oxygenic phototrophs and many anoxygenic ones.

·        Photoheterotrophs: 

Convert light into energy but require organic compounds from their environment to make their own food.

 Oxygenic Phototrophs

These organisms produce oxygen as a byproduct of photosynthesis. Eg, Cyanobacteria, eukaryotic microalgae

 1. Cyanobacteria: Often referred to as blue-green algae, these prokaryotes were crucial in forming Earth's oxygen-rich atmosphere. 

·        Cyanobacteria can perform oxygenic photosynthesis – producing oxygen from CO₂ and water. Due to their chlorophyll pigments, they are typically greenish blue in color and therefore also known as blue-green algae. They are found in a variety of aquatic and terrestrial habitats, including even extreme locations like hot springs and deserts.

·        Cyanobacteria play a crucial role in the global carbon cycle and have had a significant impact on the evolution of our planet's atmosphere. As one of the oldest organisms on Earth, they were responsible for releasing oxygen into the atmosphere, which initiated the transformation of the atmosphere and created the environment in which we live today.

·        Cyanobacteria are commonly used in research, both as model organisms for studying photosynthesis and as potential sources of biofuels and other useful compounds.

 2. Eukaryotic Microalgae: These are microscopic, single-celled eukaryotic organisms that perform oxygenic photosynthesis. eg. diatoms, dinoflagellates. green algae etc

 Eukaryotic microalgae perform photosynthesis to convert sunlight, carbon dioxide, and water into carbohydrates (like sugars) and oxygen. 

Photosynthesis takes place inside specialized organelles called chloroplasts, which are the site of both light and dark reactions. Chlorophyll, a green pigment, is crucial for absorbing sunlight. Other pigments may also be involved depending on the specific microalgae species.  Oxygen is a significant byproduct of the light-dependent reactions, released into the environment. The carbohydrates produced are the building blocks for vital organic molecules, including lipids and proteins, forming the microalgae's biomass. 

Microalgae are highly efficient photo-synthesizers and play a critical role in the global carbon cycle and oxygen production. 

Algal Blooms

Under conditions of high nutrient availability from sources like agricultural runoff or wastewater, these algae undergo rapid overgrowth, known as an algal bloom. These blooms can discolor water and, when the algae die, their decomposition consumes oxygen, creating "dead zones" harmful to other aquatic life, though some blooms are harmless

Excessive levels of nutrients, particularly nitrogen and phosphorus from human activities like farming and wastewater discharge, lead to proliferation of microalgae, resulting in an algal bloom. Besides nutrient enrichment, climate change can also cause blooms.

Blooms can be recognized by the visible change in the water's color due to the algae's pigments. As the microalgae in the bloom die and decompose, the process consumes large amounts of dissolved oxygen resulting in the death of fish and other aquatic life.  Some algal blooms, known as harmful algal blooms (HABs), produce potent toxins that can harm or kill other organisms and negatively impact human health and economies. 

 

Anoxygenic Phototrophs

These organisms perform photosynthesis without producing oxygen, as water is not their electron donor. eg, purple bacteria, green bacteria

 

1. Purple Bacteria:  Also known as Rhodospirillineae, purple bacteria uses bacteriochlorophyll and can be further divided into purple sulfur and nonsulfur bacteria. 

Purple bacteria, are a diverse group of phototrophic bacteria that perform anoxygenic photosynthesis, which means they do not produce oxygen. They are called purple bacteria because their main pigments (bacteriochlorophyll pigment a or b located on chromatophores and plasma membranes) give them a purple or red color. They are further divided into: the purple sulfur bacteria and the purple non-sulfur bacteria.

·       The main difference between sulfur and non-sulfur purple bacteria is the electron donor they use during photosynthesis. Purple sulfur bacteria use reduced sulfur compounds, such as hydrogen sulfide or thiosulfate, as electron donors for photosynthesis. In contrast,  purple non-sulfur bacteria use organic compounds, such as lactate or succinate, as electron donors.

·         Purple sulfur bacteria use sulfide or thiosulfate as their electron donor during photosynthetic pathways. They oxidize sulfide to elemental sulfur, which accumulates as internal globules or granules within the cell. The sulfur deposition occurs inside the bacterial cell.  The purple sulfur bacteria can be used to reduce the concentration of harmful compounds like methane and hydrogen sulfide.

·       Sulfur purple bacteria are usually found in environments where sulfur compounds are abundant, such as hot sulfur springs, swamps, and sediments, while non-sulfur purple bacteria are found in a wider range of environments, including freshwater ponds and lakes, soils, and microbial mats.

Eg., Allochromatium and Thiocapsa

 

2.     Green Bacteria: This group, including the Chlorobiineae, uses bacteriochlorophyll but has different pigments than purple bacteria.

·       Green sulfur bacteria are anoxygenic photosynthetic bacteria with a unique photosynthetic apparatus adapted to low light and anaerobic conditions. Most of them are nonmotile and obligate anaerobes. 

        Their name derives from their characteristic green color, which is due to the presence of chlorosomes – organelles that contain bacteriochlorophyll pigments. They have bacteriochlorophyll pigments c, d, a or e.        

·       They use sulphide as their ultimate electron donor for photosynthesis. Thus, they can thrive well in sulfur-rich environments with low light intensities. Most of these bacteria can reduce nitrogen to ammonia. This ammonia is later used to synthesise amino acids.

·       They are found in a variety of environments and can use different electron donors for photosynthesis, including hydrogen sulfide and elemental sulfur. 

·       These bacteria can synthesize large amounts of sulfur granules, which protect them from oxidative stress and give them a distinctive appearance, visible under a microscope. The sulfur granules are stored outside the cell as a byproduct of their anaerobic photosynthesis, distinguishing them from purple sulfur bacteria that store them intracellularly. 

·       They are also known for forming complex microbial communities in sulfide-rich environments, called mats or biofilms, playing crucial roles in biogeochemical cycling and ecosystem function.

Eg., Chlorobium tepidum and Chlorobium vibrioforme 

Purple Sulfur bacteria store sulfur inside their cells (intracellular storage)  allowing immediate access and faster regulation, when external availability becomes limited. Also, intracellular location protect the granules from being washed away in aquatic environments

Green Sulfur Bacteria store sulfur outside their cells (extracellular storage) thus avoiding cluttering and providing more cytoplasmic space for other metabolic activities. Extracellular storage may facilitate sharing or recycling of sulfur among nearby microbes in dense microbial mats or sediments.

    Both strategies offer adaptive advantages suited to their ecological niches, especially in anaerobic, sulfur-rich environments like lakes, sediments, and hot springs.

3.     Aerobic Anoxygenic Phototrophic Bacteria (AAPs): 

        These bacteria perform anoxygenic photosynthesis but require oxygen. They are mostly marine or freshwater genera. AAPB contain bacteriochlorophyll a as its main light harvesting pigment,. 

    Aerobic anoxygenic phototrophic bacteria are photoheterotrophic, meaning they obtain their carbon from organic compounds. They exist in a variety of aquatic environments and may constitute over 10% of the open ocean microbial community. AAPB are thought to play an important role in carbon cycling. 

1.      Acidobacteria and Heliobacteria 

 Heliobacteria perform anoxygenic photosynthesis and form endospore abilities, while Acidobacteria is a vast phylum with diverse metabolic capabilities, primarily found in soil.  

Acidobacteria are primarily chemoheterotrophs, though some such as Chloracidobacterium thermophilum are capable of photosynthesis. They are ubiquitous and abundant, especially in soil ecosystems, peatlands and mineral-rich environments. They are motile eg., Acidobacterium capsulatum and non-spore formers 

Heliobacteria use a unique form of chlorophyll, chlorophyll g as a light-harvesting pigment unlike other photosynthetic bacteria that use bacteriochlorophyll. They are obligate anaerobes and are typically found in anoxic environments such as freshwater sediments or soil. Heliobacteria are important members of the microbial community in these environments, where they play important roles in the cycling of nutrients and carbon. 

They are obligate anaerobes that are able to capture energy from light by photophosphorylation to produce ATP. Water is not used as an electron donor and, therefore, the production of oxygen is non-existent.

 Both phyla are important to soil ecology : 

·        Both are significant components of soil microbial communities. Acidobacteria are one of the most abundant phyla in soil, and heliobacteria are widespread in anaerobic soils.

·        Members of both groups are involved in biogeochemical cycles, including carbon and nitrogen cycling. Heliobacteria are known for their nitrogen-fixing capabilities.

·        The two phyla, show vast metabolic diversity found within bacteria. Acidobacteria possess a wide array of genes for degrading complex compounds, while heliobacteria use a unique photosynthetic process.  

Ecological Importance of Phototrophic Organisms 

Phototrophic organisms are essential for maintaining the balance of ecosystems and play a crucial role in the global carbon cycle. By converting light energy into chemical energy, phototrophic organisms produce organic matter that is then consumed by other organisms. This process forms the base of most food chains and supports the growth of all other living things in the ecosystem. 

Photosynthetic organisms are also responsible for producing the oxygen that we breathe. Oxygenic photosynthesis has played a critical role in the evolution of life on Earth, as oxygen is required by many organisms for respiration and other metabolic processes.

 In addition to producing oxygen and organic matter, phototrophic organisms also play a significant role in regulating the Earth's climate. By removing carbon dioxide from the atmosphere through photosynthesis, these organisms help to mitigate the effects of climate change.

 Phototrophic organisms have significant economic importance and are used in a wide range of applications in industry, agriculture, and medicine.

·       Plants, algae, and other phototrophs are the primary source of food for humans and many other animals. In addition, photosynthetic bacteria are used in the production of certain types of food, such as fermented dairy products and pickled vegetables. 

·       Photosynthetic organisms are also used in the production of biofuels. Biofuels, such as ethanol and biodiesel, are produced from organic matter, such as crops and algae, that have been grown using photosynthesis. These biofuels are considered to be more sustainable than traditional fossil fuels, as they produce fewer greenhouse gas emissions and are renewable.

·       In medicine, photosynthetic organisms are used in a variety of applications, such as the production of antibiotics and other pharmaceuticals. The ability of photosynthetic organisms to carry out complex biochemical reactions makes them valuable tools in biotechnology and bioengineering. They are an important area of research for the development of new and innovative applications.

 Modern photobioreactors will help to conduct meaningful research with these interesting organisms. Photobioreactors are closed systems designed to grow photosynthetic microorganisms, such as algae and cyanobacteria, under controlled conditions of light, temperature, and nutrient supply.  These systems are used for a variety of purposes, such as the production of biomass for food, feed, or biofuels, the removal of pollutants from wastewater, and the cultivation of microorganisms for research or biotechnology applications.